Presently, numerous metals and other inorganic substances are used to fabricate electrical conductors, semiconductors, electronic devices, and electromagnetic or acoustic sensors. The utility of these materials is frequently limited by such factors as weight, mechanical fragility, fabrication problems, corrosion, scarcity, and high costs.
Many organic materials have properties which overcome or minimize these problems and possess several other advantages, such as ease of fabrication into films, filaments and complex shapes and variability in molecular design. Of particular importance is the possibility with organic materials to fabricate electronic devices whose dimensions are "molecular", such as diodes, capacitors, and gates whose dimensions are in the range of 10 angstroms to 500 angstroms.
Numerous resinous compositions that conduct electricity are known. Many of them comprise an organic resin with a conductive material, e.g., a metal or graphite, dispersed in a resin. Due to a lack of chemical bonding and the discreteness of the conductive filler, the mechanical properties are not good. For the same reasons, loading the polymeric binder with sufficient filler to produce a polymeric conductor with sufficient conductivity to meet the requirements of many applications is often not possible. Further, metallic corrosion can deteriorate the conductivity of the composition. Metals and graphite are not transparent and their inclusion prevents the fabrication of a transparent conductor.
One type of conductive resin includes radical-anion salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ) which are themselves organic semiconductors. A complex salt, M.sup.+ (TCNQ).sub.2 --, is also used and is preferred on account of a higher conductivity than the corresponding simple salt, M.sup.+ TCNQ--. The properties of polymeric semiconductors of the polycation-TCNQ type have some advantages over their monomeric derivatives in that they are processable and their conductivity can be controlled by varying the TCNQ concentration. However, the matrix polymer is brittle due to its ionic nature, and its stability is lowered by sensitivity to moisture. Another approach is to disperse TCNQ salts into non-ionic matrix polymers so that advantage can be taken of the mechanical properties and higher stabilities of the polymer. For example, U.S. Pat. No. 3,679,944 relates to the dispersion of TCNQ salts, such as N-methylphenazinium TCNQ in polyamides and cellulose polymers. In that patent, solid N-methylphenazinium TCNQ particles are added to a solution containing the polymer. The patent teaches that the solvent used for dissolving the polymer must not dissolve neutral TCNQ or its salt. Unfortunately, this product still has compatability, stability and dispersion problems.
U.S. Pat. No. 4,374,048, incorporated herein by reference, relates to the incorporation of microcrystals of various TCNQ salts, for example N,N,N-triethylammonium TCNQ complex salt into a poly(vinylacetal) matrix. The composition is prepared by dissolving the polymer, TCNQ and the salt in a mutual solvent, then casting out a film from the solution. While the resulting film has excellent properties, it also has some drawbacks. Most notably, the salts referred to in that patent have a limited solubility in matrix polymers, although somewhat higher solubilization occurs in high electron donor-strength polymer matrices. Thus, the loading capacity of the polymer is limited. Furthermore, the low solubility of these salts can cause crystallization to occur too rapidly when the polymer films are cast and dried. Thus, the crystals tend to be too large and agglomerate, so that it is difficult to form a uniform, interconnecting microcrystal network. To enhance solubility, a relatively strong electron donor-matrix polymer is required. However, the polymer tends to donate an electron to the neutral TCNQ (TCNQ.degree.) component of the complex salt. thus destabilizing the complex anion, forming a polymer.sup.+ TCNQ-- complex and suppressing microcrystallization. To counteract this destabilization, since only the complex form of that salt is conductive and stable, additional TCNQ.degree. must be added to the polymer solution, forming (TCNQ).sub.2 --. Ideally, the ratio of TCNQ.degree./TCNQ-- in the polymer is 1.
In U.S. Pat. No. 3,966,987, incorporated herein by reference, an electrically conductive polymer is prepared by dissolving a complex salt of N-methylacridinium and a nitrogen containing organic polymer in a mutual solvent and casting a film from the resulting solution. However, only the complex salt of N-methylacridinium TCNQ is stable and conductive. In contrast, both the simple and complex N-methylphenazinium TCNQ salts used in this invention are stable and conductive, thus allowing both the complex and simple salt to contribute to the conductivity of the final polymer composite. Furthermore, it is not believed that the N-methylacridinium TCNQ/polymer system exhibits the unique and unexpected film morphology exhibited by the present invention. Additionally, nitrogen-containing polymers make poor matrices in the present invention, since they destabilize the TCNQ radical anion due to their generally low ionization potentials.
The major advantage of a polymer/TCNQ salt composite system in which the simple salt contributes to the conductivity is that such a system requires a lower weight percent of overall TCNQ to achieve a specified resistivity than does a polymer composite system in which only the complex salt is conductive. Since high doping levels of TCNQ salt species adversely affect polymer strength, a polymer/TCNQ salt composite system in which both simple and complex salts are conductive should, at most resistivities, have a greater mechanical strength than a polymer composite system in which only the complex TCNQ salt is conductive.